1. Field of the Invention
The present invention generally relates to a method for manufacturing a zipper and, more particularly, to a method for manufacturing a watertight zipper that can effectively prevent penetration of liquid.
2. Description of the Related Art
With references to FIGS. 1 and 2, a conventional watertight zipper 9 has a waterproof tape 91 made of an elastic material. The waterproof tape 91 has a sheet portion 91a and an enlarged edge portion 91b, with the enlarged edge portion 91b connected to an edge of the sheet portion 91a. The thickness of the enlarged edge portion 91b is larger than the thickness of the sheet portion 91, forming upper and lower shoulders 911 and 912 at the junction of the sheet portion 91a and the enlarged edge portion 91b. A free end of the enlarged edge portion 91b forms an abutting surface 913. The waterproof tape 91 includes apertures 92 spaced out and extending through the sheet portion 91a. A scoop 93 is formed at each aperture 92, with upper and lower teeth of the scoop 93 extending into the aperture 92 to connect with each other. The upper tooth of each scoop 93 abuts against the upper shoulder 911 of the waterproof tape 91, and the lower tooth of each scoop 93 abuts against the lower shoulder 912 of the waterproof tape 91. Free ends of the upper and lower teeth of the scoop 93 extend beyond the enlarged edge portion 91b, with the abutting surface 913 of the enlarged edge portion 91b sandwiched between the upper and lower teeth of the scoop 93. As such, when the conventional watertight zipper 9 is fastened and the scoops 93 of left and right stringers are engaged with each other, the enlarged edge portions 91b of the left and right stringers abut against each other via their abutting surfaces 913, thus preventing penetration of liquid through a void between the scoops 93 of the left and right stringers. An embodiment similar to the conventional watertight zipper 9 is disclosed in U.S. Pat. No. 2,910,754.
When manufacturing the conventional watertight zipper 9, an injection molding process and an extrusion process are widely used. Concerning the injection molding process, the size of the mold directly limits the length of the waterproof tape 91. That is, each mold can only produce waterproof tapes 91 in one size. When the waterproof tape 91 with a longer length is required, it must be manufactured by repeating the injection molding steps. Namely, after the initially formed waterproof tape 91 cools down, the injection molding step is repeated once more to lengthen the waterproof tape 91, and further injection molding steps may be necessary according to the demand in total length of the waterproof tape 91. Though repeating the injection molding steps can form the upper and lower shoulders 911 and 912 of the waterproof tape 91 with a plane finish and parallel to the abutting surface 913, a part of a section formed by a previous injection molding step is pressed by the mold when a later injection molding step is performed to form the next section, thus said part is deformed. Besides, since the section formed by the previous injection molding step is already solidified and the material for forming the next section by the later injection molding step is still in a liquid state, a linear protrusion usually forms at the overlap area of said adjacent sections. Consequently, penetration of liquid may easily occur at the overlap area.
Furthermore, when forming the waterproof tape 91 having a cloth tape embedded inside, since it is hard for the cloth tape to be steadily positioned inside the mold, the polymeric material cannot completely wrap around the cloth tape by the injection molding step. Hence, the waterproof tape 91 having the cloth tape embedded inside must be manufactured through the extrusion process. The extrusion process can produce the waterproof tape 91 with the cloth tape embedded inside in a continuous manner. The scoops 93 with the required size are then formed onto the waterproof tape 91 by injection molding, thus improving production efficiency. Besides, deformation or linear protrusion mentioned previously due to continuous injection molding steps will surely not occur. However, concerning the long waterproof tape 91 formed by extrusion, it is hard to form the upper and lower shoulders 911 and 912 with a plane surface and parallel to the abutting surface 913. Consequently, leakage of material may still occur during the formation of the scoops 93, which also adversely affects the yield of the scoops 93.
For this reason, a manufacturer tends to produce the waterproof tape with an even thickness by extrusion. Such a waterproof tape can be utilized for production of a continuous watertight zipper, and leakage of material during formation of the scoops can be avoided. An embodiment similar to such manufacturing method is disclosed in U.S. Pat. No. 7,337,506 B2. However, the waterproof tape having an uneven thickness can form the enlarged abutting surface with its thicker portion, resulting in an improved watertight effect. When the waterproof tape having an uneven thickness is coupled to waterproof leathers or clothes via its thinner portion, the thinner portion softer than the thicker portion can have a flexibility similar to that of the thin and soft waterproof leathers or clothes, improving the quality of the watertight product. With this concern, the waterproof tape having an uneven thickness is still a preferred choice. Hence, the conventional method for manufacturing the watertight zipper needs improvement that can solve the above mentioned problems for producing the watertight zipper with better performance.
On another aspect, with references to FIGS. 1 and 2, when the watertight zipper is fastened and the scoops 93 of the left and right stringers engage with each other, the abutting surfaces 913 of the enlarged edge portions 91b on the left and right stringers must be pressed until the abutting surfaces 913 slightly deform for achieving a sealing effect. However, since the upper and lower teeth of each scoop 93 extend along two opposite surfaces of the enlarged edge portion 91b, the distance “d” between free ends of the upper and lower teeth of each scoop 93 is the same as the width “w” of the abutting surface 913 of the enlarged edge portion 91b. As a result, when the abutting surfaces 913 of the left and right stringers abut against each other, there is no space for the enlarged edge portions 91b of the left and right stringers to slightly deform, adversely affecting its sealing function.
In light of the above, it is needed to improve the conventional method for manufacturing the watertight zipper.